1. Field of the Invention
This invention relates generally to gate valves, and more particularly, to a gate valve of the expanding gate type which includes a gate carrying one or more movable segments which move relative to the gate to effect closing and opening of the valve.
2. Brief Description of the Prior Art
Most gate valve assemblies with expanding gates have typically comprised a gate having a female V-surface formed in the upstream side of the gate, and a floating segment mounted on the gate with a complementary male V-surface which engages and coacts with the female V-surface on the gate. Such gate assembly is positioned between two seat surfaces within the valve body, and the gate assembly generally moves in a direction which is perpendicular to the conduit axis and parallel to the seat faces Sealing during opening and closing is effected by the floating segment being wedged outwardly from the gate in a direction perpendicular to the axis of the stem to effect sealing engagement with the seat faces. Such expanding gate valves may have the gate formed integrally with the stem; the gate may be connected to the stem with a floating connection; or the gate may be threadedly connected to the stem.
In an expanding gate valve of the type described, movement of the gate in a direction normal to the conduit axis brings the floating segment into contact with a stop which arrests further movement of the segment with the gate in a direction normal to the conduit axis. This causes the segment to be wedged outwardly so that the segment is brought into sealing contact with the valve seat. Such axial expansion of the segment along the conduit axis results from the wedging action of the cooperating V-surfaces carried on the segment and on the gate.
The wedging action causing axial, expanding movement of the segment occurs in the fully closed or the fully opened position of the valve, and results from predetermined, selectively located fixed stops which are interposed in the path of movement of the segment as it moves with the gate. The stops arrest further movement of the segment in this direction. When the gate is moving between the open and the closed positions, the segment is caused to collapse upon, or nest with, the gate as a result of the complementary interfitting of the engaged V-surfaces.
In order for the valve to be smoothly and easily operable, and to avoid damage to the valve stem or gate, it is necessary that the wedging action of the gate segment occurs in only the open and closed positions, and that the segment remain in the collapsed condition when the gate is moving between the open and closed positions within the valve body. This prevents unnecessary dragging or binding of the gate assembly against the valve seat faces at a time when sealing is not required. Moreover, any pre-expansion of the segment during travel, also referred to as back-wedging, may result in excessive torque applied to the stem to operate the valve, thereby causing the valve assembly to jam, or even causing failure of the stem.
In a gate assembly in which the gate is connected to the stem with a floating connection, for example with a T-head, both the gate and the segment expand axially along the conduit (fluid flow) axis at the time of closure and opening the valve. When the gate is of the non-floating type, however, because of rigid connection between the gate and the stem, only the segment can expand upon closure and opening of the valve. The restraint of the stem prevents the gate from moving axially along the fluid flow axis.
As a result of such stem restraint, a bending moment is created on the stem in the plane encompassing its axis and the conduit axis (i.e., in the direction of fluid flow). This makes the valve more difficult to operate, and presents the possibility of jamming the valve assembly. When the load on the gate and the stem is relieved, the fluid pressure in the conduit, acting on the segment and aided by the centralizing levers or torsion springs that are often employed in expanding gate valves of the type described, forces the segment to nest or collapse into the gate, thus relieving the pressure on the seat faces. The gate assembly can then travel between the position of opening and closing without dragging or binding due to unnecessary contact with the seat faces.
If the fluid pressure in the conduit on the downstream side of the gate assembly (the side opposite the side of the gate which carries the segment), or in the valve body, should build to exert excessive back pressure on the gate, the nesting or collapsing ability of the gate assembly becomes impaired, thereby causing problems as a result of back-wedging or pre-expansion of the segment. As a result of this, the valve becomes very difficult to operate, and in extreme cases, cannot be operated at all.
For the described reasons, the expanding gate valve assembly having a single V-surface on one side of the gate for the accommodation of the V-shaped male protuberance on the segment is a unidirectional assembly, and can be mounted in the valve body for operation in only one direction of fluid flow. That is, the segment must be located on the upstream or high pressure side of the valve gate in order for the gate assembly to function.
Expanding gate valves of the types described, having a gate with a movable segment mounted thereon, are shown in U.S. Pat. No. 4,189,127; U.S. Pat. No. 4,531,710; U.S. Pat. No. 4,179,009; U.S. Pat. No. 4,188,014; U.S. Pat. No. 4,188,016; U.S. Pat. No. 4,279,404; U.S. Pat. No. 4,341,369; U.S. Pat. No. 4,334,666; U.S. Pat. No. 3,823,911 and U.S. Pat. No. 4,530,488.
In substantially all expanding gate valves as such are presently constructed, the angle provided between the V-surface and the axis of the stem (the direction of opening and closing movement of the valve gate) is generally established to be of such magnitude that it aids in the collapse of the segment while at the same time the operating torque is within a practical range. As the magnitude of this angle is decreased, the ease with which the segment collapses into the gate following opening and closing is reduced, but a lower operating torque is required to open or close the valve. Conversely, the larger the angle, the more readily and easily the segment will collapse into the gate following opening and closing of the valve, but a higher operating torque is required in order to open or close the valve. So a compromise is generally made in determining the magnitude of this angle. Even then, expanding gate valves of the type described have problems associated with collapsing the segment
If the angle between the V-surface and the stem axis is not of sufficient magnitude to aid in the collapse of the segment, the gate and the segment become jammed together in the valve body, making it extremely difficult to open the valve. Therefore, in many valves, especially of the larger sizes, centralizing levers are used to aid in the collapsing or nesting of the segment. It should be mentioned here that these valves seal only in the closed position and not always in the open position. In case of hazardous fluids, for safety and environmental reasons, sealing is also required in the open position.
The present invention is designed to eliminate man of the drawbacks of expanding gate assemblies currently in use and discussed above.